In-situ preparation of sulfonated carbonaceous copper oxide-zirconia nanocomposite as a novel and recyclable solid acid catalyst for reduction of 4-nitrophenol

The missing-linker defects of UiO-66 were exploited to covalently anchor Cu nanoclusters (Cu/UiO-66). The molecular interactions between the metals and oxides as copper-zirconia interfaces in Cu/UiO-66 are essential for heterogeneous catalysis, leading to remarkable synergistic impacts on activity and selectivity. Homogeneously distributed carbonaceous mixed metal oxides (CuO/ZrO2@C) nanocomposite was prepared via carbonization of the Cu/UiO-66 at 600 °C for 3 h in air. To enhance the acidity properties of the CuO/ZrO2@C nanocomposite, a small amount of sulfuric acid was added and heated at 150 °C under an N2 atmosphere (CuO/ZrO2-SO3H@C). The synthesised Cu/UiO-66 and CuO/ZrO2-SO3H@C catalysts were used as novel catalysts in the reduction of 4-nitrophenol (4-NP) to 4-aminophenol (4-AP). The Cu/UiO-66 and CuO/ZrO2-SO3H@C catalysts displayed complete conversion of the 4-NP solution during (4 and 2 min) stirring at room temperature, respectively. These two catalysts exhibited a high reduction rate of 8.61 × 10–3 s−1, and 18.3 × 10–3 s−1, respectively. The X-ray photoelectron spectroscopic (XPS) analysis showed the charge of copper atoms in the Cu/UiO-66 catalyst was Cu0/CuII and in the CuO/ZrO2-SO3H@C catalyst was CuI/CuII with nearly the same ratio (65/35). The particle size and the elemental composition of the CuO/ZrO2-SO3H@C catalyst were analysed by using high resolution transmission electron microscopy (HR-TEM), and energy-dispersive X-ray spectroscopy (EDS), and elemental mapping, respectively. The key point beyond the high catalytic activity and selectivity of the CuO/ZrO2-SO3H@C catalyst is both the carbon–metal oxides heterojunction structure that leads to good dispersion of the CuO and ZrO2 over the carbon sheets, and the high acidity properties that come from the combination between the Brønsted acid sites from sulfuric acid and Lewis acid sites from the UiO-66. The catalysts exhibited good recyclability efficiency without significant loss in activity, indicating their good potential for industrial applications.

General procedure for the synthesis of sulfonated carbonaceous metal oxide nanocomposites. The carbonaceous metal oxides nanocomposites (ZrO 2 @C, CuO@C, and CuO/ZrO 2 @C) were sulfonated by heating in concentrated sulfuric acid (96-98 wt%) at 150 °C for 10 h under N 2 atmosphere. The nanocomposite material obtained was then washed repeatedly with hot distilled water at 80 °C until sulfate anions were no longer detected in the filtered water. Sulfonated carbonaceous metal oxides nanocomposites were finally dried in an oven at 100 °C for 2 h 32 , donated as (ZrO 2 -SO 3 H@C, CuO-SO 3 H@C, and CuO/ZrO 2 -SO 3 H@C, respectively).

Determination of the acidity of the synthesized solid acid catalysts. Potentiometric titration is
used to investigate the acidity properties of the synthesized solid acid catalysts 44 . 0.1 g of each catalyst is suspended in acetonitrile for 2 h and then titrated with n-butylamine as a base in acetonitrile (0.025 N) at a rate of 0.1 ml over each 10 min. A double-junction electrode potentiometer (Orion 420 digital) is used to measure the potential.
Catalytic activity of the synthesized catalysts. To investigate the catalytic activity of the synthesized catalysts, 50 ml of 4-NP (2 mM) was added to 50 mg of the catalysts, then 1.25 ml of NaBH 4 (2 M) was added under constant stirring 13 . The yellow color of the solution gradually transformed to colorless, indicating the reduction of 4-NP into 4-AP, during the hydrogenation process. Several cuts were directly withdrawn from the reaction medium after a certain regular interval stirring time followed by measuring using a UV-Vis spectrophotometer. The decrease in intensity of the absorption peak of the 4-NP and NaBH 4 mixture at 400 nm was followed up. The kinetic parameters of the reduction reaction were calculated. The effect of catalyst weight (50,30, and 10 mg) was studied. At the end of the reaction, the catalyst was separated from the suspension by centrifugation (6000 rpm), washed several times with water and dried for 2 h at 110 °C, and then reused to study the recyclability of the prepared catalysts. The product 4-aminophenol was also identified by capillary column gas chromatography 13 .

Results and discussion
We reported previously two methods to introduce metal nanoclusters in MOF pores 28,29 , where the metal nanoclusters were prepared by using protecting agents, such as l-cysteine 25,26 , l-glutathione 27,45 and 2-phenylethyl thiol 29,46,47 . These clusters were loaded over or in the MOFs using the simple impregnation method 29 or in-situ impregnation during the building steps of the MOFs 25 . Recently, naked copper clusters (25 ± 4 Cu atoms) were immersed in UiO-66 pores (Cu/UiO-66), where the missing-linker defects in the UiO-66 structure (~ 10%) were replaced by two -OH/OH 2 species, which are active for ion exchange to deposit metal onto the Zr 6 nodes of the MOF 1,48 . This method allows specific homotopic anchoring of naked metal clusters at the node 1,48 . According to the density functional theory (DFT) calculations, the average diameter of the copper clusters is 0.7-0. 8  Catalysts characterization. The crystallinity of the prepared catalysts was confirmed using X-ray diffraction (XRD). Figure 1-I represents the XRD pattern of the parent Cu-BDC with high intensity diffraction peaks at (2θ) 10.04°, 17.02°, and 24.70°, corresponding to the (110), (021), and (131) planes, respectively 49 , which are indicating a high crystallinity degree of Cu-BDC 43,49 . The XRD pattern of the UiO-66 shows characteristic peaks at 2θ = 7.3°, 8.5°, and 25.7°, corresponding to the (111), (200), and (531) planes, respectively ( Fig. 1-I), which are well matched with the simulated XRD pattern of UiO-66 50 . The XRD pattern of the as-synthesized Cu/UiO-66 is in excellent agreement with the XRD pattern for the parent UiO-66 28,29 , which confirms the high dispersion and lower loading percentage of the Cu nanoclusters inside the MOF's pores ( Fig. 1-I) 28 .
The prepared MOFs (Cu-BDC and UiO-66) were calcined at 600 °C in the air to prepare the carbonaceous CuO and ZrO 2 nanocomposites, respectively. The crystallinity of the nanocomposites was investigated by XRD ( Fig. 1-II 52 . The XRD pattern of the carbonaceous Cu/UiO-66 showed the same XRD pattern for ZrO 2 @C plus two diffraction peaks at 35.8° and 39° indicating the presence of CuO in the CuO/ZrO 2 @C nanocomposites ( Fig. 1-II). The prepared nanocomposites were treated with sulfuric acid at 150 °C under an N 2 atmosphere to prepare sulfonated metal oxide@carbon nanocomposites with Brønsted acid sites. The sulfonation process does not affect the crystallinity of the carbonaceous metal oxides. Figure 1-III shows the XRD diffraction patterns of the CuO-SO 3 H@C, ZrO 2 -SO 3 H@C, and CuO/ZrO 2 -SO 3 H@C nanocomposites which are similar to the carbonaceous metal oxides ( Fig. 1-II).
The N 2 adsorption-desorption isotherms at − 196 °C were used to investigate the textural properties of the prepared catalysts, as shown in Fig. 2 and summarised in Table 1. The Brunauer − Emmett − Teller (BET) equation was used to measure the specific surface areas of the prepared MOFs (Cu-BDC and UiO-66) 644 and 1315 m 2 /g, respectively, (Fig. 2-I). The S BET of the Cu/UiO-66 decreases slightly due to the partial occupation of the UiO-66 pores with the deposited Cu nanoclusters ( Table 1). The parent MOFs and loaded UiO-66 with copper clusters exhibit type I adsorption-desorption isotherms with H4 hysteresis loops according to the IUPAC classification of hysteresis loops 53 . The specific surface areas of the prepared MOFs are far higher than the carbonaceous copper and zirconium oxide ( Fig. 2-II) and sulfonated carbonaceous copper and zirconium oxide ( Fig. 2-III), as summarised in Table 1. The specific surface area of the CuO/ZrO 2 @C nanocomposite is higher than the carbonaceous copper and zirconium oxide due to the contribution of the copper species 1 .
The Barrett-Joyner-Halenda (BJH) method is used to measure the pore size distribution of the prepared catalysts ( Table 1). The total pore volume of Cu-BDC and UiO-66 is 420 × 10 -3 and 650 × 10 -3 cm 3 /g, respectively. The total pore volume of the Cu/UiO-66 catalyst decreases slightly due to the incorporation of the Cu clusters inside the UiO-66 pores (Table 1). However, there is a considerable variation in the pore volume distribution of the parent and loaded MOFs and the carbonaceous CuO and ZrO 2 nanocomposites, where the pore volume of CuO@C, ZrO 2 @C, and CuO/ZrO 2 @C nanocomposites are 77 × 10 -3 , 140 × 10 -3 and 170 × 10 -3 cm 3 /g (Table 1),  www.nature.com/scientificreports/ respectively. The specific surface areas of the prepared catalysts are measured by another method, the T-method (S t ) which shows the same values as S BET (Table1), which confirms the correct choice of the standard t-curves for pore analysis 22,24,28 . The X-ray photoelectron spectroscopy (XPS) technique is used to demonstrate the chemical composition and the charge state of the Cu nanoclusters in the prepared nanocomposites [28][29][30][31] . Figure 3-I displays the survey XPS spectrum of the Cu/UiO-66 that indicates the catalyst contains four elements Zr 3d, 4p, 3s and 3p, Cu 2p, C 1s, and O 1s. The high resolution Cu 2p XPS spectrum was shown in Fig. 3-Ia, the two peaks at binding energies 932.6 and 934.4 eV that corresponding to Cu 3p 3/2 were related to Cu (0) and Cu (II), respectively 31,54 . Another two peaks appear at binding energies of 952.5 and 954.3 eV, which correspond to Cu 3p 1/2 and related to Cu (0) and Cu (II), respectively 54 . The peak position and the corresponding satellites indicate exists of CuO ( Fig. 3-Ia). The satellite peaks originate from numerous excitations in copper oxides 54 . The Cu 0 /Cu II ratio in the Cu/ UiO-66 catalyst is 65/35%, which is in good agreement with the X-ray absorption near edge structure (XANES) technique 1 . Figure 3-Ib displays the Zr 3d 5/2 and 3d 3/2 peaks, which are observed at 182.8 eV and 185.1 eV, respectively, and are in good agreement with the published XPS spectrum for ZrO 2 55 . These peaks strongly imply that Zr 4+ is the most common oxidation state for Zr 54 Table 1. Surface area and pore volume data of the prepared catalysts.

Catalysts
S BET (m 2 g −1 ) S t (m 2 g −1 ) Pore volume (cm 3 /g) × 10 -3  www.nature.com/scientificreports/ XPS analysis is used to identify the chemical structure of the prepared sulfonated carbonaceous metal oxides (CuO/ZrO 2 -SO 3 H@C, ZrO 2 -SO 3 H@C, and CuO-SO 3 H@C). Figure 3-II shows the survey XPS spectrum of CuO/ ZrO 2 -SO 3 H@C that indicates the catalyst contains five elements Zr 3d, 4p, 3s and 3p, Cu 2p, C 1s, O 1s, and S 2p. The high resolution Cu 2p XPS spectrum confirms the CuO/ZrO 2 -SO 3 H@C has a mixture of two copper oxides (Cu 2 O and CuO) as shown in Fig. 3-IIa. The two peaks at binding energies 933 and 953 eV that corresponding to Cu 3p 3/2 and 3p 1/2 are related to Cu (I), respectively 31,54 , and another two peaks at binding energies of 934.5 and 954.5 eV, that corresponding to Cu 3p 3/2 and Cu 3p 1/2 are related to Cu (II), respectively 54 . The Cu I /Cu II ratio in the CuO/ZrO 2 -SO 3 H@C catalyst is 64/36% (Fig. 3-IIa). The high resolution XPS spectra of the Zr 2p and C 1s (Fig. 3-IIb,c) are the same as the above-mentioned peaks position of the Cu/UiO-66 catalyst, respectively. The S 2p spectrum of the CuO/ZrO 2 -SO 3 H@C catalyst shows two different peaks at binding energy 168.7 and 169.9 eV that are attributed to the S-O and S=O bonds (Fig. 3-IId). The peak separation of 1.2 eV that indicates the S in the CuO/ZrO 2 -SO 3 H@C is mainly in the form of SO 3 H groups bonded to the CuO/ZrO 2 nanocomposite 57 . Figure 4-I shows the survey XPS spectrum of ZrO 2 -SO 3 H@C that indicates the catalyst contains four elements Zr 3d, 4p, 3s and 3p, C 1s, O 1s, and S 2p. The survey XPS spectrum of CuO-SO 3 H@C exhibits also four elements Cu 2p, C 1s, O 1s, and S 2p (Fig. 4-II). The high resolution XPS spectra of these elements are as mentioned above (Fig. 4).
The HR-TEM image of the CuO/ZrO 2 -SO 3 H@C (Fig. 5-I) displays the monoclinic structure of ZrO 2 and CuO particles as confirmed by XRD analysis (Fig. 1-III). The particle size of the ZrO 2 and CuO particles over the carbon sheets are 12-25 nm and 6-10 nm, respectively (Fig. 5-I). Figure 5-II exhibits the energy dispersive X-ray spectroscopy (EDS) and elemental mapping of the prepared CuO/ZrO 2 -SO 3 H@C to determine its elemental composition. The deeper investigation of the EDS data reveals that the C, Zr, Cu, O, and S elementals were unevenly present in the CuO/ZrO 2 -SO 3 H@C, as shown in Fig. 5-II. The random distribution of these elementals (C, Zr, Cu, and S) was strongly supported from the elemental mapping (specified by different colors) of the prepared CuO/ZrO 2 -SO 3 H@C (Fig. 5) 58 .
The surface acidity of the prepared catalysts was determined using non-aqueous potentiometric titration by measuring the electrode potential variation by Orion 420 digital model. A known amount (0.1 g) of the prepared catalysts was suspended in acetonitrile for 2 h and then titrated with 0.025 N n-butylamine and the electrode potential was measured. The acid strength of the surface sites is determined by the electrode's initial potential (Ei) and the total number of acid sites are calculated from the curve plateau with mequiv/g units. The strength of the acid sites can be classified according to the following scale: Ei > 100 mV (very strong sites); 0 < Ei < 100 mV (strong sites); − 100 < Ei < 0 mV (weak sites); and Ei < − 100 mV (very weak sites) 59 . The potentiometric titration curves are presented in Fig. 6-I, which illustrates the electrode potential variation versus volume added from n-butyl amine. The initial potential (Ei) of the neat UiO-66 and Cu-BDC equals 135.6 and 100 mV, respectively, which indicates they have moderate acidity 44,60 . The binding energy of NH 3 in the undefective and defective regions of the UiO-66 is 75.8 and 110.1 kJ mol −1 per NH 3 molecule, respectively, clearly demonstrating enhanced binding at the defect center 61 . The copper clusters were bonded in the defective region to form the Cu/UiO-66, where the Cu atoms formed the Cu-O-Zr bonds that have a significant formal positive charge, therefore enhancing the Lewis acidic properties of the UiO-66 1 . Incorporation of the copper clusters inside the UiO-66 frameworks increased the acid strength of the Cu/UiO-66 (Ei = 215 mV) and created strong acid sites on the surface, where the total number of acid sites is equal to 2.3 × 10 20 mequiv g −1 . Table 2 displays an increase in the total number of acid sites on the sulfonated carbonaceous metal oxides surface in this order CuO/ZrO 2 -SO 3 H@C > CuO-SO 3 H@C > ZrO 2 -SO 3 H@C. So, the treatment of the carbonaceous metal oxides with H 2 SO 4 enhances their acidity properties. Moreover, the high acidity value of CuO/ZrO 2 -SO 3 H@C may be due to the synergetic effect between copper and zirconium metals. The total number of acid sites/g of the prepared catalysts was calculated from Eq. (1). (1) Total number of acid sites/g = steady point of plateau * equiv./g * N A /100  Filiz studied the effect of support material on the catalytic reduction of 4-NP using CuO nanoparticles, the supports were ordered as follows: ZrO 2 > Al 2 O 3 > SiO 2 > CaO > MgO > ZnO 65 . In this work, we prepared a carbonised CuO/ZrO 2 @C nanocomposite via calcination of Cu/UiO-66 at 600 °C in air. CuO/ZrO 2 @C shows high catalytic activity in the complete conversion of 4-NP into 4-AP within only 10 min of stirring in comparison to CuO@C and ZrO 2 @C (Fig. S1). The carbon sheets of CuO/ZrO 2 @C nanocomposite provide efficient adsorption of 4-NP due to the functional groups of carbon, such as non-covalent interactions including π-π stacking, hydrogen bonds, and so on 62 .
Recently, Mhlwatika et al. prepared a series of perovskite materials (ABO 3 ) as heterogeneous catalysts in the reduction of 4-nitrophenol, the activity of these catalysts does not depend on the surface area but depends on the acidic strength 66 . UiO-66, MOF-5 (Zn-BDC), and MIL-101 (Fe-BDC) were used to activate the reduction of 4-nitrophenol to 4-aminophenol, the result indicates that UiO-66 exhibited the best catalytic behavior due to its Lewis acidic nature at the metal nodes 64 . Moreover, many MOFs were used as catalysts in different reactions due to their unique properties [67][68][69] . Therefore, we planned to enhance the catalytic activity of the carbonaceous metal oxides by treating them with sulfuric acid under an N 2 atmosphere. Sulfonated carbonaceous metal oxides (ZrO 2 -SO 3 H@C, CuO-SO 3 H@C, and CuO/ZrO 2 -SO 3 H@C) exhibited amazing catalytic activity in the reduction of 4-nitrophenol ( Fig. 7-III). The CuO/ZrO 2 -SO 3 H@C catalyst succeeded in the reduction of the 4-nitrophenol solution into 4-aminophenol within only 2 min of stirring at room temperature ( Fig. 7-II). The catalyst has very strong acid sites according to the classification (Ei > 100 mV) 59 , where its initial potential (Ei) is equal to 366 mV Table 2. Acidity values of the prepared catalysts.  www.nature.com/scientificreports/ and the total number of acidic sites is equal to 3.05 × 10 20 mequiv g −1 ( Table 2). The reason for the effect of the acid sites is still under investigation. However, we expect that the acid sites permit better adsorption of nitrophenol on the catalyst surface and promote the breakage of the N-O bond in the intermediate phenylhydroxylamine, thus facilitating the reactions. Furthermore, the generated protons from the hydrolysis of NaBH 4 do not strongly adsorb on the acidic surface, they are readily available to participate in the conversion of 4-NP 66,70 . The effect of catalyst weight (50,30, and 10 mg) on the reduction of 4-NP was investigated ( Fig. 8 and Fig. S2). The time of the reaction was increased with the decrease of the catalyst loading. The time to complete conversion of 4-NP is 2 min, 4 min, and 8 min over 50 mg, 30 mg, and 10 mg of the CuO/ZrO 2 -SO 3 H@C, respectively ( Fig. 8-II).
We chose the Cu/UiO-66 and CuO/ZrO 2 -SO 3 H@C catalysts to calculate the rate constant of the reduction reaction using the equation ln(C t /C 0 ) = ln(A t /A 0 ) = − kt, where C t is the concentration of 4-NP at time t, C 0 is the initial concentration, and k is the apparent rate constant (Fig. 9). As a result, the reaction process can be described as a pseudo-first-order reaction in terms of 4-NP concentration. The constant rates of Cu/UiO-66 and CuO/ ZrO 2 -SO 3 H@C are 8.61 × 10 -3 s −1 and 18.3 × 10 -3 s −1 , respectively ( Fig. 9-I-II). The two catalysts were also used to study reusability ( Fig. 9-III). The reusability of Cu/UiO-66 and CuO/ZrO 2 -SO 3 H@C has been investigated several times (Fig. 9-III). The catalysts can be reused up to five times without losing their catalytic performance 71 . The catalytic efficiency of Cu/UiO-66 and CuO/ZrO 2 -SO 3 H@C catalysts is nearly 100% without significant loss in activity during the five cycles ( Fig. 9-III).
The mechanism for the reduction of 4-NP into 4-AP is presented in Fig. S3. Firstly, the sodium borohydride produces hydrogen, which is adsorbed on the catalyst surface, and then the reduction process happens. The reduction process requires six protons (6H + ) to convert the nitro group (NO 2 ) to an amino group (NH 2 ).  [74][75][76] and mixture from two noble metals (Au and Ag) 77 . According to the results in Table 3, our prepared catalysts are more effective and cheaper. 100% conversion of the 4-NP solution over our catalysts was achieved    (Table 3). Our catalysts are cheap materials compared to other catalysts that contain noble metals and expensive supports such as dendrimers 74 and graphene oxide 75 .

Conclusions
The missing-linker defects in the UiO-66 were exploited to incorporate copper clusters inside the MOF pores through covalent bonding with the Zr 6 O 8 nodes via Cu-O-Zr bridges. The received catalyst (Cu/UiO-66) was used to prepare CuO/ZrO 2 @C nanocomposite via carbonization at 600 °C for 3 h in air. The acidity properties of the nanocomposite were enhanced by doping with sulfuric acid. The CuO/ZrO 2 -SO 3 H@C nanocomposite displayed a complete conversion of the 4-NP into 4-AP within only 2 min of stirring at room temperature with a high reduction rate of 18.3 × 10 -3 s −1 . A combination between the Lewis and Brønsted acid sites on the catalyst's surface is the reason for its high catalytic activity. Moreover, the carbon sheets act as a protective agent for the prepared mixed oxides. As a result, the prepared catalysts can be reused for several cycles without significant loss of catalytic activity. Therefore, the catalysts have a high potential for the reduction of nitro compounds under mild reaction conditions.

Data availability
All data generated or analyzed during this study are included in this published article (and its Supplementary Information files).